1. Field of the Invention
The present invention relates to a focusing apparatus provided in a digital camera, a video camera, etc.
2. Description of the Related Art
As a method for detecting a focus state of an image pickup lens, Japanese Patent Laid-Open No. (“JP”) 2008-52009 discloses an apparatus configured to provide a pupil splitting focus detection using a two-dimensional sensor in which a micro lens is formed for each pixel of the sensor. In this apparatus, a photoelectric converter of each pixel in the sensor is divided into a plurality of sections, and each divided photoelectric converter receives light from a different pupil area of the image pickup lens via the micro lens. A correlation is calculated between a pair of output waveforms of the photoelectric converters that have received light from different pupil areas of the image pickup lens, and a defocus amount is calculated from the phase difference.
However, such a focus detecting method in which the light from the different pupil area of the image pickup lens is received via the micro lens, is influenced by light shielding by the image pickup lens due to the optical performance of the micro lens. When the light is shielded by the image pickup lens, a difference between in the pair of output waveform shapes used for the correlative operation increases an error contained in the calculated defocus amount. In addition, since the light shielding by the image pickup lens varies according to manufacturing errors, the unsteadiness of the apparatus, and variations with time, etc., it is difficult to remove a harmful influence on the focus detection caused by the light shielding, only by making adjustments concerning the light shielding in the manufacture.
For this harmful influence caused by the light shielding, JP 2008-52009 discloses a focusing apparatus configured to calculate a final defocus amount using a defocus amount obtained with a different diaphragm aperture diameter in the image pickup lens. When a plurality of candidate defocus amounts are derived from an output signal with a certain diaphragm aperture diameter, JP 2008-52009 again receives light with a different diaphragm aperture diameter, and calculates a defocus amount from the obtained output signal. A first calculated defocus amount candidate is compared with a second calculated defocus amount candidate with each other, and an approximately equal defocus amount is selected as a correct defocus amount. Thus, the reliability of the defocus amount is determined based upon a plurality of defocus amounts each obtained with a diaphragm aperture diameter.
It is known that as the calculated defocus amount increases, the detecting error generally increases. In order to reduce the influence of the detecting error when a detected defocus amount is large, there is known a method of resuming a focus detection by driving a lens with a driving amount smaller than that for the detected defocus amount. According to this method, the lens gradually approaches to the in-focus position with reduced influence of the detecting error.
However, the prior art disclosed in JP 2008-52009 needs to repeat the step of receiving light and of calculating a defocus amount a plurality of times so as to obtain a highly reliable defocus amount, requiring a long time for focusing.
In addition, the above control with a lens driving amount smaller than that for the detected defocus amount increases the number of detections of the defocus amount, requiring a long time for focusing. This problem can be solved if the reliability of the detected defocus amount can be determined. However, if the prior art disclosed in JP 2008-52009 continues focusing until the in-focus state is obtained, a longer time becomes necessary.